Field
One or more exemplary embodiments relate to a method and system for improving control of a steam generator level for preventing oscillation of the steam generator level in a nuclear power plant, and more particularly, to a system and method of preventing oscillation of a steam generator level and resultant shutdown of a nuclear reactor, which may be caused when a high-level priority control function is frequently and repeatedly turned on/off as the steam generator level is excessively increased, by improving a feedwater control system in the nuclear power plant.
Description of the Related Art
A nuclear power plant consists of 100 or more systems respectively having individual functions. The systems are largely classified into a nuclear steam supply system (NSSS) that is based on a nuclear reactor, a turbine-generator system which is supplied with steam to operate a generator so as to produce electricity, and other subordinate facilities.
A nuclear power plant with a pressurized water reactor (PWR), which is currently mainly used as a nuclear power plant in Korea, consists of a primary system that is based on a nuclear reactor, a secondary system that includes a turbine, a steam generator, an electric generator, and a condenser, an engineered safeguard system that is to prepare for accidents, a power transmission and supply system, an instrumentation and control system, and other subsidiary systems.
Hot water generated by a nuclear reactor circulates through a heat-transfer pipe included in a steam generator, which is connected to the nuclear reactor through a coolant pipe, so as to transfer heat to feedwater flowing into the steam generator through another pipe, and then, returns to the reactor. A level of the feedwater in the steam generator should be appropriately maintained so that the steam generator easily performs this function.
A nuclear power plant generates a signal for shutdown of a nuclear reactor according to an increase or a decrease in a steam generator level so as to ensure safety.
However, if a transient event occurs in a nuclear power plant, since a shutdown margin of a nuclear reactor having a low steam generator level is relatively less than a shutdown margin of a nuclear reactor having a high steam generator level in a feedwater control system, there is a high possibility of shutdown of a nuclear reactor when a steam generator level is high.
Accordingly, a feedwater control system in the related art has a high-level override (HLO) function such that, if a steam generator level is equal to or higher than 85%, as shown in FIG. 1, all feedwater control valves are closed and a main feedwater pump is operated at a lowest speed by setting an output value of a feedwater control system to “0”.
The HLO function is performed so that a high-level priority control mode is executed if a steam generator level is 85%, and then, deactivated if the steam generator level reaches 80%.
However, in a feedwater control system in a related art, if a signal for controlling a steam generator level is drastically switched to an output signal from a proportional integral controller when a high-level priority control mode is deactivated due to recovery of a steam generator level, since a high-level priority control function is frequently turned on/off as the steam generator level is changed, the steam generator level may oscillate. Thus, shutdown of a nuclear reactor having a high steam generator level may occur. Accordingly, a method of controlling a steam generator level to prevent oscillation of a steam generator level while a high-level priority control function is being performed is needed.